1. Field of the Invention
This invention relates to a signal processing apparatus making use of a so-called noise shaping circuit to reduce the quantization noise.
2. Description of the Prior Art
Noise shaping is a technology according to which the quantization noise produced at the time of quantizing or rounding data such as audio or video signal data is changed in its spectrum so that the noise level in the audio range is reduced significantly to improve the signal to noise (S/N) ratio. To implement such noise shaping, a feeding back of quantization error components to the input side is employed, as disclosed for example in "Adaptive Noise Spectral Shaping and Entropy Coding in Predictive Coding of Speech" appearing in IEEE Transactions on Acoustics, Speech and Processing, February, 1979, vol. ASSP -27, No. 1, pages 63 to 73. However, when an audio signal, for example, is subjected to such noise shaping, it may occur that a characteristic pattern, so-called idling pattern, which is rather harsh to the ear, is produced at minute signal levels.
The conventional practice to overcome such inconvenience is to perform noise shaping after a dither signal is added to an input signal, as disclosed in the Japanese laid open Patent Publication No. 51-48214, and as shown herein in FIG. 6.
In this figure, an input signal supplied to an input terminal 61 is transmitted to an adder 62 where it is added to a dither signal from a dither generator 63 before being transmitted to a noise shaping circuit 64. The output from the noise shaping circuit 64 is transmitted to a D/A (digital-to-analog) converter 65 and thereby converted into corresponding analog signals which are taken out at an output terminal 68.
FIG. 7 shows the frequency spectrum of the output signals from an output terminal 68. In this figure, X stands for an input signal, D a dither signal introduced at the input side and V.sub.N a quantization error component. By noise shaping, the quantization noise component V.sub.N shows a lower noise level for the lower frequencies in the audio range. The output signal Y from the output terminal 68 is represented by the following formula (1) EQU Y=X+D+V.sub.N ( 1)
Hence, an inconvenience arises that the dither signal D is supplied as an output even though there is no input signal, that is the input signal X is zero. Meanwhile, the signal to quantization noise power ratio or S/N ratio is given by the formula (2) EQU S/N=X.sup.2 /V.sub.N.sup.2 ( 2)
It is also necessary that the frequency range of the introduced dither signal be outside the audio range, while it is also necessary that the dither signal remaining at the time of D/A conversion be removed by a low pass filter.